shelf?

[mr31]

can somebody please describe what a shelf is when talking about the mainsail?

[Faster]

On bolt-rope footed mains, with the outhaul and flatteners eased there was a designed-in flat area along the boom that was the "shelf". Theory is it creates an "end plate" effect and increased the efficiency of the sail. As the outhaul and flattener are tensioned, the shelf kind of disappeared into the foot as the material was stretched out.

With todays trend toward loose footed mains the shelf is not so prevalent anymore.